The deadly deep freeze enveloping the Midwest sent temperatures plunging Wednesday to rival some of the most frigid spots in the world, triggering widespread closures of schools and businesses, and the cancelling of more than 1,600 flights from Chicago’s airports.

The U.S. Postal Service even took the rare step of suspending mail delivery across much of the region. Hundreds of public schools and universities from North Dakota to Pennsylvania cancelled classes as residents huddled inside amid one of the coldest air masses in years.

The bitter cold is the result of a split in the polar vortex that allowed temperatures to drop much farther south than normal. That meant temperatures in parts of the Midwest were lower Wednesday than in Antarctica, where the Amundsen-Scott South Pole Station hit negative 25 degrees (negative 31.7 Celsius) — balmy compared to Fargo, North Dakota’s negative 31 degrees (negative 35 Celsius) and Minneapolis’ negative 27 degrees (negative 32 Celsius), according to the National Weather Service.

Snowplows were idled overnight because of the cold in southwestern Minnesota, where temperatures dropped to negative 29 degrees (negative 34 Celsius). In Chicago, temperatures were still dropping after plunging early Wednesday to minus 19 degrees (negative 28 Celsius), breaking the day’s previous record low set in 1966 — and colder than the weather in Barrow, Alaska, the most northern town in the U.S.

And that doesn’t include wind chill, which in northern Illinois made the air feel as cold as negative 57 degrees (negative 49.4 Celsius). The National Weather Service warned that a wind chill of minus 25 (negative 32 Celsius) can freeze skin within 15 minutes.

Officials throughout the region were focused on protecting vulnerable people from the cold , including the homeless, seniors and those living in substandard housing. Some buses were turned into mobile warming shelters to help the homeless in Chicago.

“These (conditions) are actually a public health risk and you need to treat it appropriately,” Chicago Mayor Rahm Emanuel said Tuesday. “They are life-threatening conditions and temperatures.”

About 1,300 of Wednesday’s cancelled flights in Chicago were at O’Hare International Airport, one of the nation’s busiest airports. United Airlines spokesman Charlie Hobart said “everything tends to slow down” during severely cold weather, including manpower, fueling and equipment. Calling the temperatures “dangerous,” Hobart said United was bringing in extra workers and providing heated tents for employees.

A popular saying goes: “Neither snow nor rain nor heat …” will stop the mail from being delivered, but extreme cold did so Wednesday. The U.S. Postal Service has suspended mail delivery in parts or all of several Midwest states.

Governors in Illinois, Wisconsin and Michigan declared emergencies as the worst of the cold threatened on Wednesday. In Chicago, major attractions closed because of the bitter cold, including the Lincoln Park Zoo, the Art Institute and the Field Museum.

At least six deaths have been linked so far to the weather system, including two people in the Detroit area.